1. Field
The present teachings relate to electronic switches, and particularly to a circuit and method for controlling charge injection in semiconductor-on-insulator (SOI) radio frequency (RF) switches.
2. Description of Related Art
Radio frequency (RF) switches for directing RF signals are found in many different RF devices such as televisions, video recorders, cable television equipment, cellular telephones, wireless pagers, wireless infrastructure equipment, and satellite communications equipment. As is well known, the performance of RF switches is controlled by three primary operating performance parameters: insertion loss, switch isolation, and the “1 dB compression point.” The “1 dB compression point” is related to, and is indicative of, the linearity performance of an RF switch. Linearity performance is also indicated by the levels of RF signal harmonics generated by an RF switch, particularly at high RF power levels. These three performance parameters are tightly coupled, and any one parameter can be emphasized in the design of RF switch components at the expense of others. A fourth performance parameter that is occasionally considered in the design of RF switches is commonly referred to as the switching time or switching speed (defined as the time required to turn one side of a switch on and turn the other side off). Other characteristics important in RF switch design include ease and degree (or level) of integration of the RF switch, complexity, yield, return loss and cost of manufacture.
Charge injection is a problem that may occur in switching circuits such as SOI RF switches. Charge injection occurs when an applied voltage, such as a gate bias voltage, is connected to a “resistively-isolated node” through a coupling capacitance. A resistively-isolated node is defined herein as a node that at some interval during operation is connected to other circuit elements only through very high resistance connections. For example, a resistively-isolated node may occur at a transistor channel node located between series-connected transistors when the transistors are in an OFF-state. The coupling capacitance through which charge injection occurs may be either a parasitic capacitance of a circuit element (e.g., gate-to-source capacitance in a transistor), or a capacitance associated with a capacitor. For example, a bias voltage applied to the gate of a transistor may be connected to a resistively-isolated source node of the transistor through the gate-to-source capacitance. In general, charge injection may be a problem for many types of switching circuits. In particular, charge injection is significantly deleterious to the performance properties of SOI RF switches. Further, teachings on prior art SOI RF switches do not address this problem, for reasons described in more detail hereinbelow. Consequently, a need exists for a novel circuit and method for controlling charge injection in SOI RF switches.